Prep the pan: Heat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment, leaving an overhang for easy lifting. Lightly grease the parchment.

Melt the butter: In a medium saucepan over low heat, melt the butter until smooth. Let it cool for 3–4 minutes so it’s warm, not hot. This helps avoid scrambling the eggs.

Whisk in sugars: Add brown sugar and granulated sugar to the warm butter. Whisk until glossy and well combined. The mixture should look thick and shiny.

Add eggs and vanilla: Whisk in the eggs one at a time, then add vanilla. Mix until the batter looks smooth and slightly lighter in color.

Combine dry ingredients: In a separate bowl, whisk flour, baking powder, baking soda, and salt. Stir the dry mixture into the wet ingredients with a spatula until just combined. Do not overmix.

Fold in add-ins: Add your chosen mix-ins. Keep the total to about 1–1 1/2 cups so the bars stay soft and chewy.

Spread and smooth: Scrape the batter into the prepared pan. It will be thick. Use a spatula to spread it evenly into the corners. For extra shine, lightly dampen the spatula and smooth the top.

Bake: Bake for 20–26 minutes, depending on your oven and add-ins. The edges should be set and lightly golden, and the center should look slightly underbaked but not jiggly.

Optional finish: Immediately sprinkle a pinch of flaky sea salt over the top to balance the sweetness.

Cool and slice: Cool in the pan for at least 30 minutes. Lift out using the parchment and cut into bars. For clean edges, chill the pan for 20 minutes before slicing.